What it means
An attribution window defines: if a user sees or clicks an ad, and converts within X days, the ad gets credit for the conversion. If they convert after X days, the conversion is unattributed.
Common windows: 1-day click, 7-day click, 7-day click + 1-day view, 28-day click. Meta and Google offer different defaults and let advertisers configure. The shorter the window, the stricter the attribution; the longer, the more generous (and potentially overstated).
Why it matters
Different windows tell radically different stories about the same campaigns. A campaign that looks profitable on 7-day-click attribution might be a money-loser on 1-day-click. A view-through window inflates ROAS by crediting purchases that the ad arguably had no causal role in.
The fix is consistency: pick a window for the type of business you run (long sales cycle = longer window, impulse buy = shorter), apply it across every platform, and benchmark against itself over time. Do not compare ROAS at different windows.
Example
A B2B brand with a 21-day average sales cycle was using 1-day-click attribution because that was Meta's default. Most of their genuine ad-driven revenue was being unattributed. Switching to 28-day-click revealed the campaigns were 3x more profitable than reported. Budget allocation followed.